Structure for and method of tube expansion



P 1968 E. P. ,HABDAS 3,380,271

STRUCTURE FOR AND METHOD OF TUBE EXPANSION Filed July 12, 1965 .5Sheets-Sheet 1 IO 9 S z Q I n w I '9 I ,9;

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ATTORNEYS April 30, 1968 E. P. HABDAS 3,380,271

STRUCTURE FOR AND METHOD OF TUBE EXPANSION Filed July 12. 1965 5Sheets-Sheet 2 76 3- f5 5; s if -:g :5 "5' I 5- a: 5-. 58 22 Wm w-v FIG.5 INVENTOR. I w P. HABDAS BY m) ATTORNEYS April 30, 1968 E. P. HABDAS3,330,271

STRUCTURE FOR AND METHOD OF TUBE EXPANSION Filed July 12, 1965 .3Sheets-Sheet 3 VACCUM DRAWING APPARATUS INVENTOR. EDWARD P. HABDAS ATTOR NEYS United States Patent 3,380,271 STRUCTURE FOR AND METHOD OF TUBEEXPANSION Edward P. Habdas, Dearborn, Mich., assignor to Calumet &Hecla, Inc., Allen Park, Mich., a corporation of Michigan Filed July 12,1965, Ser. No. 470,980 19 Claims. (CI. 72-56) ABSTRACT OF THE DISCLOSUREStructure for expanding plain sections of finned tubing or the like,including a die having an annular internal recess for positioning overthe section of the tubing to be expanded, a source of electric energy,means connected to the source of electric energy and adapted to beextended into the tubing to the section thereof to be expanded forproviding a mechanical tube expanding force on the section of the tubeto be expanded in response to a signal from the signal source and suchmethod of tube expansion. In one modification the means for providing anexpanding force on the section of tube includes a radially expandablehollow coil haviiig a conduciing liquid therein connected to the sourceof electrical energy and means for concentrating the flux lines at theinner diameter of the coil on energizing the coil. In a secondmodification the means for providing a tube expanding force comprises aliquid within the tube and means for initiating an electric are at thesection of tube to be expanded in response to a signal from the signalsource.

The invention relates to structure for and a method of expanding tubesand the like and refers more specifically to a method of expandingfinned tube plain sections by high energy rate techniques and structurefor practicing the method.

In many applications of finned tubes it is required that plain sectionsbe provided on a finned tube which have the approximate outer diameterof the tube fins. With fins of low height the fins can be produced bysinking the tube to reduce the internal diameter thereof whereby plainsections may be left with an outer diameter substantially equal to theouter diameter of the fins. With higher fin heights the sinkingtechnique is difficult or impossible due to a tendency of tubes tocollapse when deep sinking is attempted.

Therefore, where high fin heights are required along with plain sectionsof a diameter slightly greater than the fin diameter, alternate methodsare generally used. One such method is to expand the plain ends of thetube with any of a variety of well-known mechanical expansion tools.However, the expansion of plain sections between finned sections alongthe length of the tube is difficult to achieve with known mechanicalexpansion tools. Another method for supplying the expanded ends is toweld or braze on sections that have been formed to the required shape ina separate operation. Again, this method is not readily adaptable tointermediate plain sections and is a relatively costly operation.

Electromagnetic forming of tubes has been previously known and is verysuccessful with the coil producing the electromagnetic force positionedexteriorly to a tube that is to be reduced in diameter. However, whenthe coil is positioned within the tube so that it will produce radiallyoutward forces on the tube as required for expansion of its diameter thedesign of the coil becomes more difiicult, especially in the range of 1"diameter and below Where existing internal coils are limited torelatively low energy input. This limitation is generally due to thedesign problems encountered in providing sufiicient structural strengthto withstand coil destructive forces for a reasonable life span and inproviding adequate cooling for high pulse repetition rates.

It is therefore one of the objects of the present invention to providean internal coil of unique and improved design for expanding plainsections of finned tubes.

Another object is to provide improved structure for expanding plainsections of tubes.

Another object is to provide an improved method of expanding plainsections of tubes.

Another object is to provide structure for electromagnetic expansion ofa plain section of a finned tube including a die having an annularrecess therein positioned around the plain section to be expanded, asource of high energy rate electric signals and means within the tube atthe plain section connected to the source of electric signals forproducing a radially outward force on the plain section in response toan electric signal from the signal source.

Another object is to provide structure for electromag netic expansion ofa plain section of a finned tube as set forth above wherein the meansfor producing a radial force on the tube includes a coil positionedwithin the plain section to be expanded which is expandable radiallyoutwardly on receipt of a high energy rate electric signal.

Another object is to provide structure as set forth above wherein theexpandable coil is wound with a hollow, resilient, electricallyinsulating material such as rubher which is filled with an electricallyconducting material capable of a specific amount of nondestructiveexpansion, such as liquid mercury or specially braided copper.

Another object is to provide structure as set forth above wherein a coreof high strength material having radially extending flanges at both endsthereof is provided on which the expandable coil is mounted.

Another object is to provide structure as set forth above wherein theouter surface of the central portion of the core and the axially innersurfaces of the flanges at the end of the core are coated with amaterial which is a good conductor of electricity.

Another object is to provide structure for the electrohydraulicexpansion of a plain section of a finned tube as set forth above whereinthe means for producing a radial force on the tube includes a shock wavetransmitting fluid within the tube and means connected to said source ofelectric signals for producing an electric are within the fluid filledtube at the plain section thereof.

Another object is to provide a method of expanding plain sections offinned tubes including the steps of placing a die having an annularrecess therein around the plain section of the tube, creating a highenergy rate electric signal, and producing a mechanical force operableon the interior of the plain section of the tube in response to the highenergy rate electric signal to expand the plain section of the tube intothe annular recess.

Another object is to provide a method of expanding plain sections offinned tubes as set forth above wherein the electric signal is passedthrough a fiexi'b-le coil positioned within the tube at the plainsection thereof and the coil is expanded into contact with the tube inresponse to the electric signal to provide the mechanical force.

Another object is to provide a method as set forth above and furtherincluding the step of filling the tube with a shock wave transmittingfluid medium and producing an arc Within the tube at the section to beexpanded in response to the electric signal to produce a shock wave inthe fluid which provides the mechanical force.

Another object is to provide structure for and a method of expandingplain sections of finned tubes and the like which is simple, economicaland etficient.

Other objects and features of the invention will become apparent as thedescription proceeds, especially when taken in conjunction with theaccompanying drawings, illustrating a preferred embodiment of theinvention, wherein:

FIGURE 1 is an elevation view of structure for practicing a method ofexpansion of a plain section of a finned tube constructed in accordancewith the invention.

FIGURE 2 is an enlarged view of a portion of the structure illustratedin FIGURE 1 showing die members in position around the plain section ofa finned tube to be expanded before expansion of the plain section.

FIGURE 3 is an enlarged view of a portion of the structure illustratedin FIGURE 1 similar to the view of FIGURE 2 but with the die members inan open position and the plain section of the finned tube expanded.

FIGURE 4 is an enlarged view of a portion of the die structureillustrated in FIGURE 1 showing the finned tube broken away and showingan expandable electromagnetic forming coil in longitudinal crosssection.

FIGURE 5 is a reduced diagrammatic section view of the expandableelectromagnetic forming coil of FIGURE 4, illustrating the concentrationof the flux lines between the inner surface of the coil and highlyelectrically conductive core coating and the spreading of the flux linesbetween the coil and a poorly electrically conducting tube and diemembers produced in response to a pulse of electric energy received bythe coil.

FIGURE 6 is a partly broken away view of an expandable electromagneticfOrming coil similar to that illustrated in FIGURES 2 and 3 and showingthe coil constructed of especially braided metal such as copper.

FIGURE 7 is an elevation view of modified structure for expanding aplain section of a finned tube.

FIGURE 8 is an enlarged broken away view of a portion of the tubeexpanding structure illustrated in FIG- URE 7.

With particular reference to the figures of the drawings, one embodimentof the present invention will now 'be considered in detail.

As shown in FIGURES 1-5, the structure 10 for expanding plain sections12 of finned tube 14 includes the supporting structure 16 for the finnedtube 14 and the die structure 18 for providing an annular die recessinto which the plain sections 12 of the finned tube 14 may be expanded.A source of high energy rate electric signals 20 and an expandableelectromagnetic coil 22 connected to the signal source by a coaxialcable 60 completes the tube expanding structure 10.

In operation, with the finned tube 14 supported on the supportingstructure 16 and with the die structure 18 forming an annular die recess24 around a plain section 12 of the finned tube, the coil 22 isenergized by an electric signal from the signal source 20. The coil 22expands radially into contact with the section 12 of the finned tube 14with which it is aligned to expand the section of the tube into theannular die recess 24.

More specifically, the supporting structure 16 for supporting the finnedtube 14 may include a longitudinally extending platform 26 having thesupporting rollers 28 mounted thereon for rotation by pivot means 30.The finned tube 14 is placed on the rollers 28 which are provided inpairs and inclined to be in line with radii of the finned tube 14 forsupport of the tube during longitudinal movement thereof. Suchsupporting structures for finned tubes are well known and will nottherefore be further considered herein.

The die structure 18 includes the upper and lower die members 32 and 34each having a semi-circular internal recess 36 and 38 respectivelytherein. The semi-circular recesses '36 and 38 form the annular recess28 with the die members 32 and 34 closed about a plain section 12 of thefinned tube 14, as shown in FIGURE 2.

The die members 32 and 34 are supported on piston rods 40 and 42 formovement between a closed position,

.4 as shown in FIGURE 2, and an open position, as shown in FIGURE 3, onactuation of the piston and cylinder structures 44 and 46, respectively.Both hydraulic and pneumatic means for actuating the piston and cylinderstructures 44 and 46 are well known and will not be considered in detailat this time.

The source of high energy rate electric signals 20, shown in FIGURE 1,includes a charging circuit 48 for charging a capacitor 50 when switch52 is closed. On closing of the switch 54 with capacitor 50 charged, thecharge on the capacitor 50 is placed across the electrodes 56 and 58 ofthe coaxial cable 60. The switches 52 and 54 may of course be actuatedmanually or automatically and other means for providing a pulse ofelectric energy across the conductors 56 and 58 of the coaxial cable 60may be substituted for the specific source of high energy rate electricsignals 20 indicated.

The electromagnetic coil 22, as best shown in FIGURE 4, includes ahollow helical tube 62 of expandable material, such as rubber, havingwear characteristics to permit extended use thereof as a die formingmember. As shown in FIGURE 4, the convolutions of the hollow tube seaton each other to form a helical coil. The hollow tube 62 is filled witha fluid medium 64 which is a conductor of electricity such as mercury.The mercury is in communication at the opposite ends of the tube 62 withthe conductors 56 and 58 of the coaxial cable 60. In the modified coil22 shown in FIGURE 6 the mercury is replaced with braided copper wire 65which will expand and contract in length.

The tube 62 is wound on the core 66 of high strength material, such assteel, between the radially extending flanges 68 and 70 of the core 66as shown best in FIG- URE 4. A thin layer or coating of highlyconductive material, such as copper 72, is provided on the radiallyouter surface 67 of the central portion of the core 66 and on theradially extending axially inner surfaces 69 and 71 of the radiallyextending flanges 68 and 70 of the core 66 between the core 66 and thecoil 22. The conductors 56 and 58 extend through insulating sleeves 74,76 and 78 in core 66, as shown in FIGURE 4.

In operation, when it is desired to expand a plain section 12 of thetube 14, the tube 14 is positioned on the supporting structure 16 withthe plain section to be expanded in alignment with the die members 32and 34 which during the positioning of the tube 14 are open, asillustrated in FIGURE 3. The die members 32 and 34 are then closedaround the plain section 12 of the tube 14 to be expanded, as shown inFIGURE 2, and a coaxial cable 60 connected to an expanding coil 22, asshown best in FIGURE 4, is inserted through the end 80 of the tube 14into alignment with the section 12 of the finned tube 14 which it isdesired to expand.

The coaxial cable 60 is connected to the source of high energy rateelectric signals 20 with the switch 54 open. The switch 52 is closed tocharge the capacitor 50 from the charging circuit 48. The switch 52 isthen opened and the switch 54 closed to provide a pulse of electricenergy through the expanding coil 22.

On a high energy rate electric signal passing through the mercury 64 ofthe expanding coil 22, flux lines 72, as shown in FIGURE 5, are producedabout the coil 22 which are concentrated at the layer of highconductivity material 72 positioned between the core 66 and coil 22 dueto a high resistance to transmission of flux lines produced by the coil22 through the high conductivity material 72 offered by the material 72.

The high concentration of flux lines adjacent the radially inner surfaceof the expandable coil 22 will provide a force proportional to thedensity of the flux lines and their distance from the coil 22 to causeradial expansion of the convolutions of the coil 22 outwardly to expandthe plain section 12 of the tube 14 into the annular recess formedbetween the die members 32 and 34 on contact of the coil tube 62 withthe inner surface of the plain section of the tube.

The coil 22 is also subjected to an axial force tending to compress thecoil during the radial expansion thereof due to the provision of theflanges 68 and 70 on the core and the coating of high conductivitymaterial on the axially inner surface thereof and the consequentconcentration of fiux lines at the ends of the coil, as shown best inFIGURE 5.

Since the flux lines which are exterior of the coil tend to resistradial outward expansion of the expandable coil, concentration of theexterior flux lines close to the radially outer surface of the coil isundesirable. Thus, expansion of highly electrically conductive materialsis not as readily accomplished by the structure illustrated in FIGURES14 as is the expansion of materials of relatively poor conduction whichallow the magnetic field produced by the coil to travel outward withalmost undiminished velocity and negligible flux concentration exteriorto the coil.

To reduce the magnitude of the radially inward forces on the coil due tothe concentration of flux lines external to the coil when anelectrically conductive tube'to be expanded has been positioned adjacentto the radially outer surface of the expandable coil, an expandable coilhaving a relatively large outer thickness of the insulating expandablematerial of which the coil tube 62 is mademay be used. Thus since thesame magnitude of flux appears inside coil 22 as outside coil 22, adifference in flux density is obtained since the insidefiux exists overthe small cross-sectional area between the inside of the coil 22 and theelectrically conductive coating 72 while the outside flux exists over arelatively larger cross-sectional area between the outer surfaces of theconductive core 64 and the inner surface of the tube 12 which is beingexpanded. Since the forces tending to radially expand the coil which areproportional to the inside flux density squaredare made considerablygreater than the forces tending to radially reduce the coil which areproportional to the outside flux density squared, a net radially outwardforce is produced which accelerates the coil radially outward. The highenergy rate electrical pulse is of such short duration that it has endedby the time the coil has expanded sufficiently to change the fluxdensity unbalance significantly. The work of expanding the tube is thusperformed by the inertial effects when the high velocity coil contactsthe tube.

After expansion of the coil due to a high energy rate electric signalreceived thereby, the coil will resume its normal position, asillustrated best in FIGURE 4, wherein the convolutions thereof arepositioned on the core 66.

The modified structure 90 illustrated in FIGURES 6 and 7 for expandingplain sections of finned tubes comprises a supporting structure 92having rollers 94 thereon on which a finned tube 96 is supported in aposition inclined to the horizontal, as shown in FIGURE 6. The structure90 further includes the die members 98 and 100 again providing anannular recess 102 into which a plain section of the finned tube 96 maybe expanded. The piston and cylinder structures 106 and 108 are providedas part of die structure 104 for reciprocating the piston .rods 110 and112 to which the die members 98 and 100 are secured respectively.

The die members 98 and 100 further include a gasket seal 146 around theareas contacting the tube 96 and areas mating with one another which iscapable of maintaining vacuum conditions around the tube sectionenclosed by the die members 98 and 100. Die member 100 further includesa passage 14! and fitting 143 for joining the enclosed space within theclosed dies and tube to a vacuum pumping system 144.

The vacuum conditions within the die space around the tube are desirableto allow the tube to expand against the die contour at the extremevelocities encountered in the electro-hydraulic high energy rateprocess. Without vacuum conditions the air trapped within the die cavitywould be compressed so rapidly as to cause detrimental heating of theair and a cushioning effect upon the tube.

Air vent strucure 114 is provided at end 116 of the finned tube 96. Thestructure 114 includes the sealing plug 118, air vent valve 120 and aconnecting conduit 122.

At the other end 124 of the finned tube 96 fluid filling apparatus 126is provided to fill the tube 96 with shock wave transmitting fluid, suchas water. The fluid filling apparatus 126 includes a valve 128 foralternatively connecting the water line 130 and drain line 132 to thetube 96 through sealing plug 134 and conduit 136.

A coaxial cable 138 extends through the sealing plug 134 and into thefinned tube 96 from a source of high energy rate electric signals 140which may be the same as the source of electric signals 20 illustratedin FIGURE 1. As shown in FIGURE 7, the coaxial cable 138 terminatesadjacent the die members 98 and 100 and an arc initiating wire 142 isconnected across the conductors of the coaxial cable over substantiallythe entire length of the plain section of the tube to be expanded.

In operation the finned tube 96 is positioned so that the plain sectionto be expanded is aligned with the annular recess 102 formed by the opendie members 98 and 100 at which time the cylinders 106 and 108 areactuated to clamp onto the tube and the vacuum pumping cycleautomatically or manually initiated to evacuate the recess 102. Thecoaxial cable is inserted in tube 96 and the end aligned with theannular recess 102 formed by the die members 98 and 100 with thearc-initiating wire within the section of tube to be expanded. Seal 134is then tightened into tube and around the coaxial cable and valve 128actuated to fill the tube 96 with water with the air therein beingpurged through the air vent structure 114.

A high energy rate electric signal is then fed through the coaxial cablefrom the signal source 140 whereby an electric arc is produced withinthe water filled tube at the section to be expanded. The are in theWater will cause a shock wave which will expand the tube 96 into theannular recess formed in the die. While a subsequent pressure wavefollows the initial shock wave created by the electric are formed in thewater, most of the forming it is believed is produced by the shock wave.

The water may then be drained from the tube 96. The die members areretracted and the tube 96 removed or repositioned 0n the supportingstructure 92.

While one embodiment of the present invention and a modification thereofhave been disclosed in detail, it will be understood that otherembodiments and modifications are contemplated by the inventor. It isthe intention to include all embodiments and modifications as aredefined by the appended claims within the scope of the invention.

What I claim as my invention is:

1. Structure for expanding plain sections of finned tubing or the likecomprising die structure forming an annular internal recess into whichit is desired to expand a section of a tube extending into the diestructure, a source of electric signals including a pair of conductorsextending into the tube within the section thereof to be expanded andmeans operable between the inner surface of the section of the tube andthe signal source for providing a mechanical tube expanding force on thesection of the tube in response to a signal from a signal source toexpand the tube section into the recess in the die structure, comprisinga radially expandable electric coil including a hollow helix ofexpandable material and an electric conductor within the hollow helixwhich is capable of alternate extension and retraction in lengthconnected to the pair of conductors at the opposite ends thereof.

.2. An expandable electric coil comprising a hollow helix of expandablematerial wound on a core of high strength material having radiallyextending flanges at the opposite ends thereof extending over the endsof the coil, a fluid within the hollow helix which is a conductor ofelectricity and a layer of high electric conductivity material providedbetween the coil and core, whereby the velocity of propagation ofelectromagnetic waves toward the core would be considerably reduced tocreate an area of high magnetic flux density between the coil and thelayer of material.

3. Structure for expanding plain sections of finned tubing or the likecomprising die structure forming an internal recess into which it isdesired to expand a section of a tube extending into the die structure,a source of electric signals including conductor means extending intothe tube within the section thereof to be expanded and a radiallyexpandable electric coil, the opposite ends of which are connected tothe conductor means of the signal source positioned within the tubesection to be expanded for providing a mechanical tube expanding forceon the section of the tube to be expanded in response to a signal fromthe signal source.

4. Structure for high energy rate electromagnetic expansion of a plainsection of a finned tube or the like comprising a supporting structurefor the tube, opposing die members together forming an annular recessinto which it is desired to expand the section of the tube, means forclamping the die members around the section of the tube into which it isdesired to expand the tube section, a source of high energy rateelectric signals, electrical conductor means connected to the signalsource and extending into the tube to the section thereof to be expandedand a radially expandable electric coil connected across the conductormeans, including a core, means connecting the conductor means to theopposite ends of the coil and a layer of material positioned between thecore and the coil which is a good conductor of electricity forconcentrating the flux lines produced on energization of the coilclosely adjacent the inner diameter of the coil, whereby on energizationthe coil will be caused to expand radially into contact with the innersurface of the tube section to be expanded, to expand the tube sectioninto the recess provided by the die members.

5. Structure for the high energy rate expanding of a plain section of afinned tube or the like electro-hydraulically comprising structure forsupporting the finned tube in an inclined position with respect to thehorizontal, a pair of die members having semi-circular recesses thereinpositioned on opposite sides of the section of the finned tube to beexpanded, means for advancing the die members into contact with thesection of the finned tube to be expanded to form an annular recess intowhich the section of the finned tube is to be expanded, a fitting forclosing one end of the finned tube and allowing the escape of airtherefrom, a fitting for closing the other end of the finned tubepermitting entry into and drainage from the finned tube of shock wavetransmitting fluid and entry into the tube of conductor means, a sourceof high energy rate electric signals, conductor means connected to thesource of electric signals extending through the fitting at the otherend of the finned tube and into the plain section of the tube, an arcinitiating means extending for substantially the length of the plainsection of the finned tube to be expanded connected to the conductormeans within the tube section whereby in response to a signal from thesignal source an arc is produced axially of the plain section of thefinned tube and a shock wave results which proceeds radially outwardlyinto contact with the inner surface of the plain section of the finnedtube to expand the tube section into the annular recess provided by thedie members. i

6. Structure as set forth in claim 1 wherein the electric conductor is abraided wire of high electrical conductivity.

7. Structure as set forth in claim 1 wherein the electric conductor is afluid.

8. Structure as set forth in claim 7 wherein the fluid is mercury.

9. Structure as set forth in claim 1 wherein the hollow helix is rubbertubing, rectangular in cross section.

10. Structure as set forth in claim 1 wherein the expandable coil iswound on a core of high strength material having radially extendingflanges at the opposite ends thereof extending over the ends of thecoil.

11. Structure as set forth in claim 10 wherein a layer of highelectrical conductivity material is provided between the coil and thecore whereby the velocity of propagation of electromagnetic waves towardthe core would be considerably reduced to create an area of highmagnetic flux density between the coil and the layer of material.

12. A radially expandable electric coil comprising a hollow helix ofexpandable material relatively fixed axially and a fluid within thehollow helix which is a conductor of electricity where-by on passage ofelectric energy through the fluid substantially entirely radialexpansion of the coil will occur.

13. Structure as set forth in claim 12 wherein the hollow helix isrubber tubin rectangular in cross section.

14. Structure as set forth in claim 12 wherein the fluid is mercury.

15. Structure as set forth in claim 12 wherein the expandable coil iswound on an axially relatively fixed core of high strength materialhaving radially extending flanges at the opposite ends thereof extendingover the ends of the coil.

16. Structure for high energy rate electromagnetic expansion of a plainsection of a finned tube or the like comprising a supporting structurefor the tube, opposing die members together forming an annular recessinto which it is desired to expand the section of the tube, means forclamping the die members around the section of the tube to form theannular recess into which it is desired to expand the tube section, asource of high energy rate electric signals, a coaxial cable connectedto the signal source and extending into the tube to the section thereofto be expanded and a radially expandable coil connected across theconductors of the coaxial cable, including a core of high strengthmaterial having radially extending flanges at the opposite ends thereof,a hollow, rectangular, helical, expandable tube supported on the corebetween the end flanges thereof, fluid within the hollow tube which is aconductor of electric current, in-

' sulated means connecting the conductors of the coaxial cable to theopposite ends of the coil in electric contact with the fluid therein anda layer of material positioned between the core and the coil which is agood conductor of electricity for concentrating the flux lines producedon energization of the coil closely adjacent the inner diameter of thecoil whereby on energization the coil will be caused to expand radiallyinto contact with the inner surface of the tube section to be expanded,to expand the tube section into the recess provided by the die members.

17. Structure for the high energy rate expanding of a plain section of afinned tube or the like electro-hydraulically comprising structure forsupporting the finned tube in an inclined position with respect to thehorizontal, a pair of die members having semi-circular recesses thereinpositioned on opposite sides of the section of the finned tube to beexpanded, means for advancing the die members into contact with thesection of the finned tube to be expanded to form an annular recess intowhich the section of the finned tube is to be expanded, means fordrawing in vacuum between the tube and recess, a fitting for closing theupper end of the finned tube and allowing the escape of air therefrom, afitting for closing the lower end of the finned tube permitting entryinto and drainage from the finned tube of shock wave transmitting fluidand entry into the tube of a coaxial cable therethrough, a source ofhigh energy rate electric signals, a coaxial cable connected to thesource of electric signals extending through the fitting at the lowerend of the finned tube and into the plain section of the tube, an arcinitiatin wire extending for substantially the length of the plainsection of the finned tube to be expanded between the conductors of thecoaxial cable whereby in response to a signal from the signal source anarc is produced axially of the plain section of the finned tube and ashock wave results which proceeds radially outwardly into contact withthe inner surface of the plain section of the finned tube to expand thetube section into the annular recess provided by the die members.

18. A method of expanding a section of a tube comprising positioning anannular recess in a die into wh'ch it is desired to expand the sectionof the tube around the section of the tube, inserting a flexiblesubstantially only radially expandable coil into the tube to the sectionto be expanded and providing a high energy rate electric signal throughthe coil to cause substantially only radial expansion of the coiloutwardly into contact with the inner surface of the tube section andconsequent mechanical con tact expanding of the tube section into thedie,

10 19. A method as set forth in claim 18 and further including placing acore within the expandable coil and concentrating the radially innerflux produced on passing a high energy rate electric signal through thecoil at the exterior of the core, thereby to increase the force ofoutward radial expansion of the coil.

References Cited UNITED STATES PATENTS 730,847 6/1903 Gilder et a1.174-95 1,493,713 5/1924 Tykocinski 336-20 1,748,927 2/1930 Kiemer 174-952,252,919 8/1941 Eckard 336-20 3,203,212 8/1965 Simichich 72-563,206,845 9/1965 Crunip 72-56 3,256,846 6/ 1966 Keinanen 72-56 3,267,7808/1966 Roth 72-56 3,286,497 11/1966 Cary 72-56 RICHARD J. HERBST,Primary Examiner.

